Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period. It is known for its fossils, and is mainly comprised of basalts interspersed with siliciclastic sediments.Zhou, Z. (2006). "Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives." Geological Journal, 41: 377-393. History Japanese occupation It was initially recognized during the time the Japanese Empire controlled a large chunk of Manchuria (Northern China) called "Jehol". During the occupation of Jehol, many Japanese scientists had noticed fossil remains of extinct fish and reptiles, possibly the champsosaurs. These initial fossil discoveries made by Japanese scientists vanished from history once World War II ended in 1945. Chinese rediscovery By 1949, when administration of the area passed to the Communist Party and its leader Mao Zedong, the fossils of Yixian were studied only by Chinese scientists. It was not until the 1990s when remarkable fossils of birds and dinosaurs were excavated. Since 1996, a number of dinosaur fossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-avian theropods with feathers. See Jehol Biota. Dating For some time, the formation was believed to be from the Late Jurassic-Early Cretaceous boundary, some 145 mya (million years ago). Radiometric dating has since resolved it to be younger; it is now considered to have been deposited in the Barremian to early Aptian, some 125-121 mya.Swisher, Carl C., Wang, Yuan-qing, Wang, Xiao-lin, Xu, Xing, Wang, Yuan. (1999). "Creatceous age for the feathered dinosaurs of Liaoning, China". Nature 400:58-61 1 July 1999. The Yixian Formation forms the lowest part of the Jehol group, defined by Gu (1962 and 1983) as a group of geological formations including the Jehol Coal-bearing Beds, the Jehol Oil Shale Beds, and the Jehol Volcanic Rocks. Gu, Z.W. (1983) "On the boundary of non-marine Jurassic and Cretaceous in China" in: "Nanjing Institute of Geology and Paleontology, Academica Sinica" "Stratigraphical Chart in China with Explanatory Text" Science Press, Beijing 1983:65-82. The Yixian Formation is preceded by the older Daohugou Beds, of uncertain Jurassic or Early Cretaceous age, which are sometimes considered part of the Jehol group. The Yixian Formation (including the synonymous Jingangshan, Tuhulu, Jianchang, Lower Volcanic and Volcanic Rock formations) is followed stratigraphically by the slightly younger Jiufotang Formation and the Fuxin Formation.Sha, Jingeng. (2007) "Cretaceous Stratigraphy of northeast China: non-marine and marine correlation" Cretaceous Research 28(2) pp.146-170April 2007 Chiappe et al. argued in 1999 that the oldest beds of the Yixian (those bearing a fauna dominated by confuciusornithid birds) were best separated as a distinct formation, called the Chaomidianzi Formation, with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City.Chiappe, L.M., Ji, S.A., Ji, Q., and Norell, M.A. (1999). "Anatomy and systematics of the Confuciusornithidae (Aves) from the Mesozoic of North-eastern China." Bulletin of the American Museum of Natural History, 1999. However, this classification has fallen out of favor, and the Chaomidianzi Formation is disused as a synonym of the Jianshangou Bed of the Yixian Formation.Chen, P., Q. Wang, H. Zhang, M. Cao, W. Li, S. Wu, and Y. Shen. (2005). "Jianshangou Bed of the Yixian Formation in west Liaoning, China." Science in China Series D - Earth Sciences, 48: 298-312. doi: 10.1360/04yd0038 The Dabeigou Formation in Fengning, Hebei Province may immediately precede the Yixian, or it may be equivalent to the oldest Yixian beds. The Yixian also overlies the Tuchengzi Formation in places.Wang, Y., Ken, S., Zhang, W. and Zheng, S. (2006). "Biodiversity and palaeoclimate of the Middle Jurassic floras from the Tiaojishan Formation in western Liaoning, China." Progress in Natural Science, 16(1): 222 — 230. DOI: 10.1080/10020070612330087 The Yixian Formation is divided into the following subunits, from most recent to oldest: * Jingangshan Bed * Dawangzhangzi Bed * Jianshangou Bed (124.6 Ma) * Lujiatun Bed (~125 Ma) Paleoecology The Yixian Formation represents the second of three major faunal phases that characterize the Jehol Biota, mainly based on changes in invertebrate diversity. In the Yixian, ostracods (seed shrimp) had diversified considerably, despite a very low diversity in the earlier Dabeigou Formation. Other major invertebrate groups in the Yixian include clam shrimp and insects. Insects, as a group, experienced their largest diversification of the entire Mesozoic era in the Yixian. On the other hand, some invertebrate groups, such as bivalves and gastropods (snails and slugs), were numerous but low in diversity, being mainly represented by one or two dominant species (Arguniella in the case of the bivalves). Studies of vertebrates have shown support for the division of the Jehol into phases, and the diversity of fish in the Yixian was distinct from older and younger formations, with Lycoptera as the dominant species. The Yixian preserves the first Jehol dinosaurs and pterosaurs (which have not been found in the older Dabeigou Formation), and the first major radiation of birds (only one bird species is known from the Dabeigou). The Yixian also preserves the largest (and only) mammal radiation so far known from the Jehol group. Most vertebrates showed a tendency to climb trees or become arboreal, including many tree-dwelling birds, and climbing mammals and lizards. Plant life reached its Jehol biota peak in the Yixian. Four species of flowering plant were present (three of Archaeofructus and one of Sinocarpus), as were a variety of horsetails that closely resembled modern species. It is possible that increasing animal and plant diversity were linked. The Yixian is characterized by extensive forests, dominated by trees such as ginkgoes, conifers, cycads, and seed fern trees. Ground cover plants included lycopods, horsetails, ferns, and primitive flowering plants, which were rare compared to the others. This plant life grew around a series of freshwater lakes, provided with abundant minerals thanks to periodic volcanic eruptions. Volcanic activity, along with periodic wildfires, and noxious gasses released from the lake bottoms caused the ecosystem to be continually destroyed and regrown. This, along with the wide diversity of habitats in the surrounding region, may have contributed to the fast diversification of life forms present in the Yixian ecosystem. Vertebrate fauna of the Yixian Formation Amphibians Birds and other theropods The dinosaurs listed below were compiled in a survey by Xu and Norell in 2006, unless otherwise noted.Xu, X. and Norell, M.A. (2006). "Non-Avian dinosaur fossils from the Lower Cretaceous Jehol Group of western Liaoning, China."Geological Journal, 41: 419-437. Choristoderans Fish Lizards Mammals The mammals listed below were compiled in a survey by Meng and colleagues in 2006, unless otherwise noted.Meng, J., Hu, Y., Li, C. and Wang, Y. (2006). "The mammal fauna in the Early Cretaceous Jehol Biota: implications for diversity and biology of Mesozoic mammals."Geological Journal, 41: 439-463. Ornithischians The dinosaurs listed below were compiled in a survey by Xu and Norell in 2006, unless otherwise noted. Pterosaurs The pterosaurs listed below were compiled in a survey by Wang and Zhou in 2006, unless otherwise noted.Wang, X. and Zhou, Z. (2006). "Pterosaur assemblages of the Jehol Biota and their implication for the Early Cretaceous pterosaur radiation ."Geological Journal, 41: 405-418. Sauropods Turtles Invertebrate fauna of the Yixian Formation * giant water strider bug * Jehol crayfish * Orb-weaving spider * Ephemeropsis trisetalis (a mayfly) * Carrion-eating beetles * Cicada * Dragonfly * Cockroach * Palaepangonius, an early pollinating fly , an extinct genus aquatic plants, found at the Yixian Formation.]] Flora of the Yixian Formation * ''Archaefructus * Horsetail * Ephedra * Conifer trees * Ginkgo trees * Archaeamphora, the earliest known carnivorous plant Trace fossils of the Yixian Formation See also * List of fossil sites (with link directory) * List of dinosaur-bearing fossil sites References Category:Cretaceous paleontological sites of Asia Category:Geology of China Category:Geography of Liaoning Category:Lagerstätten